The goal of this investigation was to develop stable radioimmunoconjugates (RICs) of anti-HER2/neu monoclonal antibodies (MoAbs) for imaging and therapy in an animal model bearing human breast tumor xenografts that express normal (MCF-7 cells) and increased amounts of HER2/neu receptors (HCC-1954, BT-474, SKBR-3 cells) on their cell surface membranes. Pharmacy-grade Herceptin, a murine anti-HER2/neu MoAb, and nonspecific mouse IgG protein were conjugated with the recently developed DTPA linker known as CHX-A"-DTPA. These immunoconjugates were labeled with (111)InCl(3) and (90)YCl(3). Using a molar excess of 10:1 CHX-A"-DTPA to immunoglobulin, average specific activities of 1.87 microCi (111)In/microg RIC and 2.71 microCi (90)Y/microg RIC were obtained. The purity of RICs was 96%+ for (111)In and 99%+ for (90)Y. Stability in human plasma at 37 degrees C for both RICs ranged from 98% at 24 h to 85% at 96 h. Binding capacity of the RICs was tested with human cancer cell lines MCF-7, HCC-1954, BT-474, and SKBR-3. Using (111)In-labeled nonspecific IgG protein as a control, (111)In-Herceptin RIC was found to bind to MCF-7 cells with a ratio of 2.5:1 and to SKBR-3 cells with a ratio of 85:1 after 3 h of incubation. (111)In anti-HER2/neu RIC bound to MCF-7 cells with a ratio of 6:1 and to SKBR-3 cells with a ratio of 115:1 after 3 h of incubation. (90)Y-anti-HER2/neu RIC bound 10-times greater to BT-474 cells than to MCF-7 cells. Thus, these MoAbs can be labeled with (111)In and (90)Y using the CHX-A"-DTPA linker. The resulting RICs ((111)In- and (90)Y-anti HER2/neu antibodies) are stable and bind significantly to HER2 overexpressing tumor cell lines.